Devices are known for the detection of smoke by light scatter techniques. Such devices incorporate a light source configured to irradiate a volume of air provided in a sampling region in which smoke particles may be suspended. Light scattered by said particles is collected on a light detector means. The amplitude of the signal produced from said light detector is an indication of the quantity of smoke suspended in the air.
Particularly sensitive versions of such smoke detectors are also capable of monitoring air pollution. Such high sensitivity enables detection of fires at the earliest possible (incipient) stage, whereby fires may be controlled with portable extinguishers by local personnel before smoke levels become dangerous to life. Such detectors require a sensitivity as high as 20 micrograms per cubic meter for woodsmoke, equivalent to a visual range of 40 km. To achieve such sensitivity, the light source has included a Xenon flashtube and the light detector has been a photomultiplier tube, while both devices are mounted in conjunction with a sampling chamber through which samples of airborne smoke are passed.
A prime objective of the present invention is to provide an improved smoke detector in which the disadvantages inherent with prior art devices are at least substantially overcome.
The disadvantages of the photomultiplier tubes are:
(1) being vacuum-tube devices, they are prone to breakage, damage by vibration, loss of vacuum pressure or gaseous poisoning; PA1 (2) operational life is limited; PA1 (3) care must be taken to avoid exposure to bright light such as sunlight; PA1 (4) sensitivity variation from unit to unit may be a factor of ten or more; PA1 (5) their sensitivity is affected by temperature; PA1 (6) they are of comparitively very high cost. PA1 (7) they require a costly power supply; PA1 (8) they are large and unsuitable for miniaturization.
According to one aspect of the present invention it is proposed that the photomultiplier tube of prior art devices be replaced by an extremely sensitive solid-state light detector.